The invention relates to the production of a core material for sandwich panels. Such a core material preferably consists of strips of fibrous material which are joined to one another at their surfaces with the joins offset turn and turn about In the expanded state, a pack of strips joined to one another in this way forms a honeycomb-like structure which combines low weight with high rigidity and strength in the finished panel.
A prior art method for producing such core material is disclosed in GB-A-2277709. This method for the production of a core material composed of strips of ( preferably fibrous material for a sandwich panel, in which sandwich panel the width direction of the strips is coincident with the thickness direction of the panel and a cover layer is attached to each of the longitudinal edges of the strips, comprises cutting each strip from an even number of layers with the formation of, in each case, a longitudinal edge having a cut surface.
In said finished panel the longitudinal edges of the strips are each attached to a cover sheet. The adhesion between the cover sheets and the strips of the core is obtained by means of an adhesive such as, for example, urea formaldehyde adhesive. In connection with keeping the total quantity of adhesive required as low as possible, in the case of cover sheets having a closed surface structure (such as metal or plastic) said adhesive is preferably applied to the longitudinal edges of the strips. Incidentally, in the case of cover sheets which consist of an open structure, such as fibrous material, the adhesive is applied to said cover sheets themselves.
In order to obtain adhesion of the desired strength, it is not only the quality of the adhesive that is important, but also the nature of the surfaces to be joined. The surface area of the longitudinal edges of the strips, which is determined by the cut surface which is the result of cutting off the strips from a sheet, is fairly small. Nevertheless, a certain strength of join can still be achieved because the adhesive forms beads close to the longitudinal edges on both surfaces of the strips, which beads have a beneficial effect on the join. The fact that the material of the strips is fibrous also contributes to the adhesion. The adhesive is able to penetrate into said fibrous material to some extent, as a result of which the strength of the adhesion increases.
In order to improve the absorbent capacity and the surface area of the longitudinal edges available for adhesion, the core material is fed between two rollers, which are optionally provided with a profile. Said rollers roughen the cut surfaces of the strips and as it were make said surfaces open and larger, so that a larger amount of adhesive can be absorbed and the adhesion can be improved.
The aim of the invention is to improve the production of such sandwich panels so that, on the one hand, the production process is simplified and, on the other hand, better adhesion between cover sheets and core material is obtained.
Said aim is achieved by means of providing the core material wherein the cut surface, viewed in cross-section of the strip, runs at an incline between the two surfaces of the strip.
With the method according to the invention, the strips which form the core are cut off from a sheet of starting material in such a way that an oblique cut surface is produced. As a consequence of such an oblique cut, the surface area of the cut surface is increased and specifically is increased by a factor of 1/cosxcex1, where xcex1 is the acute angle between the cut surface and the thickness direction of the strip.
Said increase in surface area yields a larger surface area of the core available for adhesion to the cover sheets, as a consequence of which the quality of the join is higher. The larger surface area is also able to absorb more adhesive.
The better adhesion is obtained on both longitudinal edges of the strips. Specifically, the strips are cut successively in a continuous process from an even number of sheets of starting material. The angles of the two longitudinal edges of a strip are therefore also complementary.
The adhesion of the honeycomb with obliquely cut longitudinal edges can be further improved by a treatment with, for example, rollers, optionally provided with a profile, as a result of which, just as in the case of conventional honeycomb, the cut surfaces are roughened.
According to the invention the angle of the cut surface can be between 20xc2x0 and 70xc2x0 with respect to the thickness direction of the strip. Preferably, said angle is approximately 45xc2x0.
The invention also relates to a method for the production of a panel from a core material and two cover layers, comprising the steps of production of a core material composed of strips of preferably fibrous material, as described above, application of an adhesive to the longitudinal edges of the strips of the core material and/or to the cover layers and pressing the core material and the cover layers onto one another.
Such panel is also disclosed in GB-A-2277709. According to the invention, during pressing the core material and the cover layers onto one another the longitudinal edges are deformed as a consequence of their inclined cut surface, which deformed a case material composed longitudinal edges provide an enlarged area of adhesion between the core material and the cover layers.
As already mentioned, the oblique shape of the longitudinal edges of the strips in the core leads to an increase in the surface available for adhesion. Said surface available for adhesion is further enlarged as a consequence of the deformation of the longitudinal edges on compressing the pack made up of core and cover sheets. This leads to a further improvement in the adhesion between core and cover sheets.
The invention also relates to a panel as produced according to the method described above.